Tibial component

ABSTRACT

A tibial knee joint prosthesis for attachment to a suitably prepared tibial bone, providing bearing portions in the lateral and medial compartments. The lateral and medial bearing surfaces of the component are inclined at different angles in the anterior to posterior direction of the knee, so that when mounted to the tibia, the lateral bearing surface of the prosthesis is higher than the medial bearing surface to the posterior side of the knee. In this way the lateral ligament is tightened progressively more than the medial ligament as the knee moves from extension to flexion, resulting in increased stability in the lateral compartment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 13/067,143filed May 11, 2011, which is a continuation of application Ser. No.12/461,435, filed Aug. 11, 2009 (U.S. Pat. No. 8,066,776), which is acontinuation of application No. 11/407,129, filed Apr. 20, 2006(abandoned), which is a continuation of application Ser. No. 10/692,016,filed Oct. 24, 2003 (U.S. Pat. No. 7,060,101), which is a continuationof application Ser. No. 10/014,920, filed Dec. 14, 2001 (abandoned),which is a continuation of PCT/GB00/02334, filed Jun. 16, 2000, whichclaims priority to British Patent Application No. 9914074.1, filed Jun.16, 1999, the entire contents of each of which are hereby incorporatedby reference.

The invention relates in general to prosthetic knee joint devices, andin particular to bi-compartmental devices for use in the lateral andmedial compartments of the tibia.

BACKGROUND OF THE INVENTION

Considerable development has taken place in recent decades with regardto knee joint replacement. However, continuous review of clinicalexperience with unicompartmental meniscal knee implants has shown thatproblems can occur, particularly in the case of replacement of thelateral compartment. Whilst great success has been achieved with medialcompartmental replacement, dislocation of the meniscal bearing in thelateral compartment remains a problem. The success rate of suchreplacements has been limited, due to the fact that the lateral softtissues (principally the lateral collateral ligament and the ilio-tibialband) offer less certain resistance to distraction of the joint. Thereis also evidence to suggest that the lateral compartment may lift offunder certain circumstances, such as during single leg stance, resultingin unequal loading of the joint. The lateral collateral ligament is amuch slimmer structure than the medial collateral ligament and there isevidence to suggest that it is slack in the unloaded joint, except atextension. It therefore offers much less resistance to bearingdislocation or lift-off than do the more inextensible medial structures.On the lateral side, the tendon of the popliteus muscle passes acrossthe postero-lateral corner of the joint. When the joint is replaced, thetendon can act to propel the meniscal bearing towards the intercondylarregion where dislocation can occur.

The early total condylar prostheses designed in the 1970's sacrificedboth cruciate ligaments as does their successor, the Insall-Bursteinposterior stabilised prosthesis. Many of the prostheses designed in the1980's sacrificed the anterior cruciate ligament (ACL) but allowedretention of the posterior cruciate ligament. Attempts to develop totaljoint prostheses where the ACL is also retained have generally beenunsuccessful. The prosthetic components were either designed toconstrain antero-posterior translational movements and inadequate stepswere taken to prevent loosening, or the components were unconstrainedrelative to anterior/posterior translation and many wore out.

It has been observed that during flexion and extension of the kneejoint, the contact points between the femoral and tibial condyles movein an antero-posterior direction. The femoral component moves on thetibial plateau in a posterior direction during flexion and in ananterior direction during extension. One of the problems caused byabsence of the ACL is increased antero-posterior movement of the femoralcondyle relative to the tibial condyle, which is responsible for furtherloosening of the tibial component and often leads to dislocation. Aconventional tibial component will generally have a large central pinfor location in the prepared end of the tibia. The ACL may be present ina diseased knee but is usually removed to improve access to the jointarea during surgery, which is necessary to provide sufficient clearanceto insert the tibial component and provide a site for the central pin.

Examination of records of the state of the ligaments at the time ofknee-replacement surgery shows that in more than 50% of cases, both inosteoarthritis or in rheumatoid arthritis, all the ligaments, includingthe ACL, were found to be intact. Where present, these ligaments aregenerally sacrificed in the above surgical techniques. Although tibialcomponents having a central cut-out slot of the type illustrated in FIG.1, which can be inserted with the ACL in place, have been employed,there will still be a tendency for dislocation or lift-off in thelateral compartment.

In those cases where the ligaments are already absent, they may bereconstructed. Since the attempts of the 1960's, the practice of ACLreconstruction in young athletes, using muscle tendons as grafts, hasbecome widespread and there is a large body of surgeons for whom thisoperation forms a substantial part of their practice. Although suchroutine reconstructions have usually only been performed in youngpersons, following injury involving ligament damage, there isconsiderable scope for reconstruction in the elderly patients who arethe usual candidates for knee replacement, in cases where the ACL isabsent.

From the foregoing it is apparent that a need exists for kneereplacement prostheses which provide sufficient stability in the lateralcompartment and which are suitable for joints with intact cruciateligaments.

SUMMARY OF THE INVENTION

The aim of the invention is to provide a tibial component that willcause the lateral ligament to be tightened relatively more than themedial. It is intended that such a component be suitable in surgicalsituations where the anterior cruciate ligament is retained, replaced orreconstructed.

In accordance with the present invention, there is provided a prosthesisfor implantation in the knee joint, said prosthesis comprising: a tibialcomponent having a first, upper surface and a second, lower surfaceopposite said first surface for attachment to the tibia, said firstsurface including a lateral bearing region and a medial bearing region,wherein the respective angles of inclination of said lateral and saidmedial bearing regions of said first surface, with respect to saidsecond surface, are dissimilar.

Advantageously, the direction of inclination is between the anteriorside and the posterior side, and the angle of inclination of the lateralbearing region is more positive than the angle of inclination of themedial bearing region. Preferably, the difference in angle ofinclination is approximately 2 to 4 degrees. The angle of the lateralbearing region may be inclined positively to the second surface and themedial bearing region substantially parallel to the second surface.Alternatively, with appropriate changes made to the angles of saw cut inthe tibia, the lateral bearing region may be substantially parallel tothe second surface, and the medial bearing region inclined at a negativeangle to said second surface, or the lateral bearing region can beinclined at a greater positive angle to the second surface, and themedial bearing region inclined at a lesser positive angle to said secondsurface. The bearing regions may be formed as flat plane plateaux. Foradded stability, the bearing regions may be given a convex,part-spherical or part cylindrical form. They can also be formed with aconvex lateral bearing region and a concave medial bearing region,similar to the physiological knee, or alternatively with either thelateral region convex and the medial region flat, or the lateral regionflat and the medial region concave. In practical terms, the angles ofinclination of said lateral and said medial bearing regions may bechosen such that, when attached to the tibia, the lateral bearing regionis inclined downwards at a lesser angle to the horizontal than themedial bearing region, in an anterior to posterior direction.

There is further provided a method of implanting a prosthesis comprisingsawing the medial and lateral compartments of the tibial bone condylesand attaching the prosthesis to the prepared surface of the tibial bone,wherein the angle of the saw cut is chosen such that the posterior sideof the lateral bearing region sits higher on the tibia than theposterior side of the medial bearing region.

In an alternative aspect of the invention there is provided a prosthesisfor implantation in the knee, said prosthesis comprising: a tibialcomponent for attachment to the tibia, having a first, upper surface anda second, lower surface opposite said first surface for attachment tothe tibia, said first surface including a lateral bearing region and amedial bearing region, arranged such that the respective angles ofinclination in sagittal planes of said lateral and said medial bearingregions of the component in situ are different.

Preferably the lateral and medial bearing regions are inclined downward,the angle of downward inclination of said lateral bearing region to thehorizontal being less than the angle of inclination of said medialbearing region, such that the posterior of the lateral bearing region ishigher than the posterior of the medial bearing region.

In a further aspect, the invention provides a prosthesis comprising atibial component having lateral and medial portions of differing height,wherein the difference in height of the lateral and medial portionsincreases posteriorly, such as to progressively tighten the lateralligament more than the medial ligament as the joint moves from extensionto flexion.

The complete prosthesis may further comprise intermediate meniscalbearing components and a femoral component for attachment to the femur.

According to another aspect of the invention, there is provided a tibialcomponent having lateral and medial bearing portions and dimensionedsuch that a difference in the respective thicknesses of the lateralbearing portion and the medial bearing portion increases in an anteriorto posterior direction.

A further aspect of the invention provides a tibial component havinglateral and medial bearing portions, wherein the thickness of at leastone bearing portion changes progressively from the anterior side to theposterior side.

Preferably the cross-sectional area of the lateral bearing portion in asagittal plane that bisects the lateral bearing portion is greater thanthe cross-sectional area of the medial bearing portion in acorresponding sagittal plane. The change in thickness of the lateral andmedial bearing portions in the anterior to posterior direction isadvantageously described by the following expression:

t _(lat(p)) −t _(lat(a)) >t _(med(p)) −t _(med(a))

Where t_(lat(p)) is the thickness of the lateral bearing portion to theposterior side, t_(lat(a)) is the thickness of the lateral bearingportion to the anterior side, t_(med(p)) is the thickness of the medialbearing portion to the posterior side and t_(med(a)) is the thickness ofthe medial bearing portion to the anterior side. A further aspect of theinvention provides a tibial component having a lateral and a medialbearing portion comprising means for tensioning the lateral ligamentsprogressively more than the medial ligament.

BRIEF DESCRIPTION OF THE DRAWINGS

To better illustrate the invention, it will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIGS. 1A, 1B and 1C illustrate a prior art tibial tray component in plan(1A), side (1B) and posterior (1C) views.

FIGS. 2A, 2B and 2C depict plan, side and posterior views of a tibialcomponent according to a first embodiment of the invention, forincorporation in a right-hand joint.

FIGS. 3A, 3B, and 3C depict plan, side and posterior views of a tibialcomponent according to a first embodiment of the invention, forincorporation in a left-hand joint.

FIGS. 4A, 4B, 4C and 4D depict plan, side and posterior views of atibial component according to a second embodiment of the invention, forincorporation in a right-hand joint.

FIG. 5 shows a side view of a tibial component according to a variant ofthe second embodiment, for incorporation in a right-hand joint.

FIG. 6 shows a side view of a tibial component according to analternative variant of the second embodiment, for incorporation in aright-hand joint.

FIG. 7 shows a perspective view of a left-hand total knee prosthesis,incorporating a tibial component according to the second embodiment ofthe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will be described in terms conventionally used to describethe geometry of the knee. The knee has an anterior side (A) andposterior side (P), and is divided laterally along sagittal planes (S)extending between the anterior and posterior sides. A central sagittalplane divides the knee into medial and lateral compartments. The tibiahas a vertical, major axis T_(M) along its length, and a horizontalplane T_(N) defined normal to its major axis. In the figures, (A)denotes the anterior side of the tibial component, (P) denotes theposterior side, (M) denotes the medial side, (L) denotes the lateralside, (S) denotes a sagittal plane, TM denotes the vertical axis of thetibia, and T_(N) the horizontal plane of the tibia.

Referring to FIG. 1, the prior art tibial component 10 comprises agenerally flat tray, shaped to correspond to the cross-sectional shapeof the top of a typically prepared tibial bone, having an anterior side(A), a posterior side (P), and medial and lateral sides (M) and (L).S_(L) and S_(M) designate typical sagittal planes, in this casebisecting the lateral and medial bearing portions. The component has afirst, upper, major surface 11, which acts as a bearing articulationsurface, and a second, lower, major surface 12, opposite the firstsurface, for attachment to the tibia. A central cutout 13, open at theposterior side (P) of the component, is provided to allow the tray to beslid into position onto a suitably prepared tibia, without beinghindered by the presence of the cruciate ligaments. To either side ofthe central cutout are defined lateral and medial bearing portionshaving flat meniscal bearing regions 14, 15 parallel to the lowersurface 12. The thickness of the component (height of upper surfacerelative to lower surface) is constant across both bearing regions.

The tibia is typically prepared by sawing the top of the tibia in eachcompartment at an angle of 7.5° sloping downwards from the horizontal,relative to the front of the tibia. A saw guide is pinned to the exposedbone of the proximal tibia and rests on a shoe around the ankle at thedistal end. The slope of the guiding surface is set at an angle of 7.5°to this guide. This guide is used to remove a sliver of bone from thetop of the tibia. The tibial component is then attached to the preparedend of the tibia oriented with its cutout facing posteriorly.

With this arrangement, since the surfaces of the lateral and medialbearing regions are parallel to one another, the lateral collateralligament is insufficiently constrained. A prosthesis with thisarrangement will have a tendency to dislocate or lift off.

The inventors have established by 3d digitisation and modelling of thehealthy knee joint that the articulating surfaces of the medial andlateral condyles of the tibia are not parallel. The medial tibialcondyle of the healthy knee joint is slightly concave while the lateralcondyle is slightly convex. Study of the digitised shapes of cadaverspecimens has enabled the positions of planar surfaces which best fitthe two condyles to be determined. A representative analysis of kneejoints shows that on average, in the sagittal plane, the angle ofposterior inclination (i.e. in an anterior to posterior direction) ofthe best-fit plane to the medial tibial condyle relative to the normalaxis of the tibia is around 2 degrees greater than the angle ofposterior inclination for the lateral tibial condyle, in other words,the upper surface of the medial condyle of the tibia slopes downwardfrom front to back at an angle on average 2 degrees greater than theupper surface of the lateral condyle.

Basing their ideas on this realisation, the inventors have developed animproved prosthetic component, which takes into account the differencein posterior inclination between the lateral and medial condyles.

FIGS. 2 and 3 illustrate right and left versions of a bicompartmentaltibial tray component 20 in accordance with a first embodiment of theinvention, similar to the conventional tibial tray component illustratedin FIG. 1. The component also has a first, upper, major surface 21,which acts as a bearing articulation surface, and a second, lower, majorsurface 22, opposite the first surface, for attachment to the tibia (notshown). By adjusting the angle of inclination in the sagittal plane ofthe lateral and medial bearing regions (24,25) to be different, thebehaviour of the prosthetic knee joint can be arranged to reduce thelikelihood of dislocation in the lateral compartment.

FIGS. 2 and 3 show the plateaux (24, 25) as flat surfaces inclined toeach other. In surgery, a single saw guide is used to saw the surfacesof the lateral and medial compartments of the tibia parallel to eachother, preferably at an angle of approximately 7.5° to the horizontalplane, normal to the major axis of the tibia. The articulating bearingsurface of the component in the medial compartment is inclined at anangle β=0 i.e. is parallel to its under surface, so will also beinclined at an angle of 7.5°, when mounted to the prepared tibia. Thearticulating bearing surface in the lateral compartment of the componentis inclined at an angle α, in the region of 3.5° to its lower surface,the component increasing in thickness in the antero-posterior direction.Thus when the component is mounted to the prepared tibia sawn at 7.5° tothe horizontal it will provide a lateral bearing surface inclined at thelesser angle of 4° to the horizontal plane, and consequently the heightdifference between the medial and lateral bearing surfaces increasesposteriorly. This will tend to keep the fibres of the lateral collateralligament tighter than physiological as the joint flexes and the femoralcondyles move backwards, increasing the contribution of the LCL toresisting bearing dislocation or lateral lift-off.

Although the relative inclination in the sagittal plane of the twobearing surfaces with the component in situ will most probably be in theregion of 2 to 4 degrees, the essential element is an appropriatedifference in slope between the bearing surfaces of the two compartmentswhen the component is in situ, such that the height of the bearingsurface on the lateral side reduces progressively at a lesser rate inthe posterior direction than the height reduces on the medial side.

In FIGS. 2 and 3, the two plateaux are shown at the same levelanteriorly and therefore at different levels posteriorly. The inventionencompasses designs in which the lateral and medial bearing surfaces areat different levels both anteriorly and posteriorly; it may beappropriate to arrange the component such that both lateral and medialbearing regions are inclined relative to the lower surface of thecomponent, but at different angles, as will be discussed later in thisspecification.

It may be appropriate to provide the tibial component with a raised sidewall or flange at one side of the intercondylar cutout, as indicated at26 in FIG. 2, in order to limit the extent of relative lateral/medialmovement of the joint and to prevent contact between the meniscalbearings or femoral condyles and the exposed cut tibial bone.

FIG. 4 (a) to (c) shows a second embodiment of the inventionincorporating a component (20) for a profiled meniscal bearing toprovide added stability. In this embodiment, by making the bearingsurfaces of convex, curved form, the resistance to dislocation can befurther enhanced. The tibial plateaux are inclined to each other in thesagittal plane as in FIG. 2, such that, in situ, a plane fitted to thesurface of the bearing region in the lateral compartment (24) will beinclined at a lesser angle to the horizontal than in the medialcompartment (25). The curved bearing regions (24, 25) shown are definedby convex surfaces of part-cylindrical shape. The surfaces can also bedefined by convex surfaces of part-spherical shape. As shown in FIG. 4(d), if only one compartment is provided with a curved surface,advantageously, less bone removal may be required. The medial bearingregion can be flat, with the lateral bearing region convex as shown inFIG. 4 (d), or alternatively, the lateral bearing region can be flat andthe medial bearing region concave. It is also envisaged that bearingsurfaces could be formed to more closely approximate the physiologicaltibial condyles, with the lateral bearing region having a convex form,and the medial bearing region having a concave form. Furtherimprovements to stability may be provided by differentially incliningthe surfaces in the frontal plane, such that each bearing region slopesdownwards from the centre of the knee towards the exterior of the knee.

The radius of curvature of the cylindrical bearing surface is a functionof the size of the prosthesis, and will increase for components destinedfor a larger knee. However, it is important that the radius be keptwithin limits, to keep the overall thickness of the tibial component asthin as possible, thus minimising the amount of tibial bone that has tobe removed to accommodate the component.

It is to be appreciated that the actual angles of the articulatingbearing surfaces of the tibial component are dependent on the angle ofthe saw cut in the tibia. A cut angle of 7.5° posterior tilt has beenfound to be ideal, especially where the femoral component is formed withspherical condyles, allowing the creation of equal flexion and extensiongaps. However, the tibial bone could be sawn at other angles, providedthat the relative change in thickness of the component in the posteriordirection was greater in the lateral compartment than in the medialcompartment, with the angles of inclination of the upper surfaces of thecomponent correspondingly selected to achieve the desired differentialinclination, when the component is in situ on the tibia. FIG. 5illustrates a possible variant to the first embodiment. The bearingregions have different slopes but are inclined in the same direction,with the lateral bearing region inclined at 5½° to the lower surface andthe medial bearing region inclined at a lesser angle of 2°. Provided thetibial saw cut is made at an increased angle of 9½° the resultingposition of the lateral and medial bearing regions in situ on the tibiawill be the same as for the first embodiment. It is to be appreciatedthat with a reduced angle of cut, in order to produce the necessarydifferential inclination, the component could even have a lateralbearing portion where the upper and lower surfaces were parallel, and amedial bearing portion where the upper surface was inclined in ananterior direction, as illustrated in FIG. 6, where lateral and medialbearing regions have angles of inclination α, β of 0° and −3½° (anegative angle indicating that the bearing region slopes toward thelower surface from the anterior side to the posterior side—as opposed toa positive angle where the bearing region slopes away from the lowersurface from the anterior to the posterior side). In this case, areduced angle of cut of 4° would be necessary to achieve the sameoverall differential inclination as in the first embodiment. It wouldalso be possible to saw the tibial condyles at different angles, withthe lateral compartment inclined at a lesser angle than the medial one,although this would necessitate a more complicated sawing arrangement.In this case a component with parallel medial and lateral plateaux couldbe employed, having a deformable central section, such that the lowersurface of the component could conform to the differentially inclinedtibial compartments, once affixed to the tibia. Where appropriate,separate lateral and medial bearing components could be employed.

FIG. 7 shows a complete left-hand 4-part knee prosthesis incorporatingthe tibial component of the second embodiment. The lateral (24) andmedial (25) bearing regions of the tibial component are intended toco-operate in use with the lower bearing surfaces of the intermediatemeniscal bearings (30,31). The upper bearing surfaces of the meniscalbearings cooperate in use with the bearing surfaces of a correspondingfemoral component (40). As the joint moves from extension to flexion,the femoral component and meniscal bearings will move backwards on thebearing surfaces of the tibial component. Due to the differentialinclination of the lateral and medial bearing surfaces, the lateralligament will be progressively tightened, providing increasingresistance to dislocation as the knee is flexed.

Whilst FIGS. 1 to 6 do not show the means for fixing the tibialcomponent to the bone, it is to be appreciated that this could beachieved in a number of ways. For example, a pin or pins could beapplied centrally at the anterior side of the prosthesis, slopingdownwards and backwards into the bone. Alternatively, pins or spikescould be applied laterally, as shown at (50) in FIG. 7. Such pins couldeither be formed as an integral part of the prosthesis, or as separateelements. Slots may be prepared in the tibia to accept the fixing means.

The component may be formed of any suitable surgically acceptablematerial, such as cobalt chrome vanadium, alumina or zirconia ceramic,or plastic, such as ultra-high molecular weight polyethylene (UHMWPE).Whilst the component described in the above embodiments is of unitaryconstruction it is to be appreciated that it could also be constructedwith separate lateral and medial bearing portions secured to one side ofa tray element, with the other side of the tray element for attachmentto the end of the tibia. Although the above embodiments have generallybeen described in the context of 3 component total knee prostheses, itis to be appreciated that the invention may also be applied with twocomponent arrangements, where the femoral and tibial components beardirectly against one another. In such arrangements, having a lateraltibial plateau which is higher posteriorly could prove particularlyuseful in reducing any problems of lift-off in extension of the joint.The tibial component in such arrangements may be formed from separateelements, as mentioned above, with bearing portions of a suitablematerial (e.g. UHMWPE) attached to a metal tray element. Thedifferential inclination may be provided by adapting the tray or theindividual bearing portions.

1. A prosthesis for implantation in the knee joint, said prosthesiscomprising: a tibial component having a first, upper surface and asecond, lower surface opposite said first surface for attachment to thetibia, said first surface including a lateral bearing region and amedial bearing region, wherein the respective angles of inclination ofsaid lateral and said medial bearing regions of said first surface, withrespect to said second surface, are dissimilar
 2. A prosthesis accordingto claim 1 wherein the direction of inclination is between said anteriorside and said posterior side.
 3. A prosthesis according to claim 1wherein the angle of inclination of the lateral bearing region is morepositive than the angle of inclination of the medial bearing region. 4.A prosthesis according to claim 1 wherein the difference in angle ofinclination between the lateral and medial bearing regions is within arange of approximately 2 to 4 degrees.
 5. A prosthesis according toclaim 1 wherein the height of the lateral bearing region increases fromsaid anterior side to said posterior side.
 6. A prosthesis according toclaim 1 wherein the lateral bearing region is inclined at a positiveangle to the second surface, and said medial bearing region issubstantially parallel to said second surface.
 7. A prosthesis accordingto claim 1 wherein the lateral bearing region is substantially parallelto the second surface, and said medial bearing region is inclined at anegative angle to said second surface.
 8. A prosthesis according toclaim 1 wherein the lateral bearing region is inclined at a positiveangle to the second surface, and said medial bearing region is inclinedat a lesser positive angle to said second surface.
 9. A prosthesisaccording to claim 1, wherein the lateral and medial bearing regions areformed as flat plane plateaux.
 10. A prosthesis according to claim 1,wherein the lateral and medial bearing regions have a convex,part-cylindrical form.
 11. A prosthesis according to claim 1, whereinthe lateral bearing region has a convex form and the medial bearingregion has a flat form.
 12. A prosthesis according to claim 1, whereinthe lateral bearing region has a flat form and the medial bearing regionhas a concave form.
 13. A prosthesis according to claim 1, wherein thelateral bearing region has a convex form and the medial bearing regionhas a concave form.
 14. A prosthesis according to claim 1 wherein theangles of inclination of said lateral and said medial bearing regionsare chosen such that, when attached to the tibia, the lateral bearingregion is inclined downwards to the horizontal at a lesser angle thanthe medial bearing region, in an antero-posterior direction.
 15. Amethod of implanting a prosthesis according to any of the previousclaims comprising sawing the medial and lateral compartments of thetibial bone condyles and attaching the prosthesis to the preparedsurface of the tibial bone, wherein the angle of the saw cut is chosensuch that the posterior side of the lateral bearing region sits higheron the tibia than the posterior side of the medial bearing region.
 16. Aprosthesis for implantation in the knee, said prosthesis comprising: atibial component for attachment to the tibia, having a first, uppersurface and a second, lower surface opposite said first surface forattachment to the tibia, said first surface including a lateral bearingregion and a medial bearing region, arranged such that the respectiveangles of inclination in sagittal planes of said lateral and said medialbearing regions of the component in situ when fitted to the tibia aredifferent.
 17. A prosthesis according to claim 16, wherein the lateraland medial bearing regions in situ are inclined downward to thehorizontal from the anterior to the posterior side, the angle ofdownward inclination of said lateral bearing region to the horizontalbeing less than the angle of downward inclination of said medial bearingregion, such that the posterior of the lateral bearing region is higherthan the posterior of the medial bearing region.
 18. A prosthesiscomprising a tibial component having lateral and medial portions ofdiffering height, wherein the difference in height of the lateral andmedial portions increases posteriorly, such as to progressively tightenthe lateral ligament more than the medial ligament as the joint movesfrom extension to flexion.
 19. A prosthesis according to claim 16further comprising an intermediate meniscal bearing component and afemoral component for attachment to the femur.
 20. A tibial componenthaving lateral and medial bearing portions and dimensioned such that adifference in the respective thicknesses of the lateral bearing portionand the medial bearing portion increases in an anterior to posteriordirection.
 21. A tibial component having lateral and medial bearingportions, wherein the thickness of at least one bearing portion changesprogressively from the anterior side to the posterior side.
 22. A tibialcomponent according to claim 21, wherein the cross-sectional area of thelateral bearing portion in a sagittal plane that bisects the lateralbearing portion is greater than the cross-sectional area of the medialbearing portion in a corresponding sagittal plane.
 23. A tibialcomponent according to claim 21, wherein the change in thickness of thelateral and medial bearing portions in the anterior to posteriordirection is described by the following expression:t _(lat(p)) −t _(lat(a)) >t _(med(p)) −t _(med(a)) Where t_(lat(p)) isthe thickness of the lateral bearing portion to the posterior side,t_(lat(a)) is the thickness of the lateral bearing portion to theanterior side, t_(med(p)) is the thickness of the medial bearing portionto the posterior side and t_(med(a)) is the thickness of the medialbearing portion to the anterior side.
 24. A tibial component having alateral and a medial bearing portion comprising means for tensioning thelateral ligaments progressively more than the medial ligament.